Cyclostrophic Wind

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Cyclostrophic Wind

In some atmospheric conditions, the radius of rotation becomes so small that the centrifugal force becomes quite strong in comparison with the Coriolis force. This is particularly true in low latitudes where the Coriolis force is quite small to begin with. In this case, the pressure gradient force is nearly balanced by the centrifugal force alone. When this occurs, the wind is said to be cyclostrophic. By definition, a cyclostrophic wind exists when the pressure gradient force is balanced by the centrifugal force alone.

This exact situation rarely exists, but is so nearly reached in some situations that the small Coriolis effect is neglected and the flow is said to be cyclostrophic. Winds in a hurricane or typhoon and the winds around a tornado are considered cyclostrophic.

Movement of Wind Around Anticyclones

The movement of gradient winds around anticyclones is affected in a certain manner by the pressure gradient force, the centrifugal force, and the Coriolis force. The pressure gradient force acts from high to low pressure, and the Coriolis force acts opposite to the pressure gradient force and at right angles to the direction of movement of the parcel of air. The centrifugal force acts at right angles to the path of motion and outward from the center about which the parcel is moving. (See fig. 3-1-12.) In the case of a high pressure center, the pressure gradient force and the centrifugal force balance the Coriolis force. This phenomenon may be expressed in the following manner:

Movement of Wind Around Cyclones

As in the case of anticyclones, gradient winds around cyclones are affected by the pressure gradient force, the centrifugal force, and the Coriolis force, but the balance of the forces is different. (See fig. 3-1-12.) In a cyclonic situation the pressure gradient force is balanced by the Cor-iolis force and the centrifugal force. This balance may be expressed in the following manner:

Centrifugal force acts WITH the pressure gra-dient force when the circulation is anticyclonic and AGAINST the pressure gradient force when the circulation is cyclonic. Therefore, wind velocity is greater in an anticyclone than in a cyclone of the same isobaric spacing.

Variations

It has been determined that, given the same density, pressure gradient, and latitude, the wind is weaker around a low-pressure cell than a high-pressure cell. This is also true for gradient and

Figure 3-1-12.—Forces acting on pressure systems.

geostrophic winds. The wind we observe on a synoptic chart is usually stronger around low cells than high cells because the pressure gradient is usually stronger around the low-pressure cell.

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